image-builder: Default to built type plain

We leave the logic for kernel downloads in place but we're almost
exclusively building plain nowadays.

build-image

@@ -126,8 +126,6 @@ EOF
 }
 
 setup_laniakea_spark_build() {
-  build_type=plain
-
   case "$distro" in
       amber)
         distro=amber-phone
@@ -145,7 +143,7 @@ cwd=$(pwd)
 distro=${DIST:-byzantium}
 mirror=
 pureos_board=devkit
-build_type=current
+build_type=plain
 NULL=
 # Device tree file name. Only set this if you're not using
 # flash-kernel and want manual uboot script configuration.